It has become increasingly popular to provide vehicles, especially motor vehicles, with sensor assemblies that provide an indication of when the vehicle is relatively close to another object. Currently, many sensor assemblies include a sonic or ultrasonic transducer. The sensor assembly actuates the transducer so that, for a fixed period of time, the transducer emits a sound wave or an ultrasonic wave. The time period in which this sound/ultrasonic wave travels away from the transducer, is reflected off an object and returns to the transducer is monitored. Based on this time period, a signal processing circuit determines the distance from the transducer to the object. In many proximity sensor assemblies mounted in vehicles, if this distance is relatively close, for example, less than 10 meters in some situations, less than 5 meters in still other situations, and less than 1.5 meters in still other situations, the signal processing circuit then actuates an audibly or visually perceptible alarm. In more sophisticated assemblies, the signal processing circuit generates a visual message that indicates the distance between the vehicle and the object.
Initially, these sensor assemblies were mounted in the rear end of large land vehicles such as trucks and buses in which it was difficult, if not impossible, for the driver to, when looking through a rear view mirror, see objects immediately behind the vehicle. Recently, these sensor assemblies have proved themselves useful in passenger cars, pickup trucks and sport utility vehicles driven by individuals. This is because some of these vehicles are relatively high off the ground. Consequently, even conscientious, careful drivers may not be able to adequately detect the presence of relatively small objects located behind the rear of the vehicle or see a child that carelessly runs behind the vehicle.
Moreover, these assemblies have proven useful because some individuals, while able to drive safely, lack the depth perception to feel comfortable moving their vehicles into confined spaces. Thus, these individuals find the presence of these proximity sensors comforting because they provide a distinct warning when the vehicle is very close to an adjacent object.
Typically, the transducer unit, or units, of these proximity sensing assemblies is mounted to the bumper or a frame element of the vehicle with which it is integral. Sometimes, for a large vehicle, the transducer is mounted to the top of the vehicle and directed diagonally downward. An advantage of this arrangement is that the transducer is out of view and therefore not an attraction to curious fingers. Moreover, sometimes these transducers are mounted so as to be oriented either substantially horizontally or diagonally upwardly. In these configurations, the proximity sensing assembly is configured to detect elevated objects, such as awnings, overhanging walls, canopies, fences or low hanging utility lines that are difficult, if not impossible, for a driver of a large vehicle to observe when they are directly behind the vehicle.
There is, however, a problem associated with currently known transducer assemblies. Owing to the nature of the function performed by these assemblies, they are exposed to the ambient environment. Consequently, rain and snow fall on these assemblies. This precipitation, be it in liquid or solid form, has a tendency to adhere to the head of the transducer through which it transmits and receives signals from the surrounding environment. Problems arise because this water, be it liquid or solid state, has a tendency to absorb and/or diffuse the energy emitted or received by the transducer. If the fraction of energy that is absorbed and/or diffused by this water is significant, the utility of their transducers diminishes and, in some instances, is rendered useless.
One solution that has been suggested to minimize this problem is to provide some sort of heating coil adjacent the transducer. A disadvantage of this arrangement is that it requires providing and mounting to the vehicle an extra component, the heating coil. This requires one to feed relatively wide gauge, large in size, power supply wires to the heating coil. Providing these power conductors adds to the expense of installing these sensors when the vehicle is being built. In the case of an older vehicle, it may be impossible to snake these conductors to the location where the sensor is to be mounted. Moreover, in both old and new vehicles, having to provide an energization current to the heating coil increases the load that is placed on the vehicle's electrical system.